System and print head for continuously manufacturing composite structure

ABSTRACT

A system is disclosed for use in additively manufacturing a composite structure. The system may include a head configured to discharge a continuous reinforcement at least partially coated with a matrix. The head may have a matrix reservoir, and a nozzle connected to an end of the matrix reservoir. The system may further include a support configured to move the head during discharging, and a supply of matrix. The system may also include at least one sensor configured to generate a signal indicative of a matrix characteristic inside of the head, and a controller configured to selectively affect the supply of matrix based on the signal.

RELATED APPLICATIONS

This application is based on and claims the benefit of priority fromU.S. Provisional Application No. 62/611,922 that was filed on Dec. 29,2017, the contents of which are expressly incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates generally to a manufacturing system andprint head, and more particularly, to a system and print head forcontinuously manufacturing composite structures.

BACKGROUND

Extrusion manufacturing is a known process for producing continuousstructures. During extrusion manufacturing, a liquid matrix (e.g., athermoset resin or a heated thermoplastic) is pushed through a diehaving a desired cross-sectional shape and size. The material, uponexiting the die, cures and hardens into a final form. In someapplications, UV light and/or ultrasonic vibrations are used to speedthe cure of the liquid matrix as it exits the die. The structuresproduced by the extrusion manufacturing process may have any continuouslength, with a straight or curved profile, a consistent cross-sectionalshape, and smooth surface finishes. Although extrusion manufacturing canbe an efficient way to continuously manufacture structures, theresulting structures may lack the strength required for someapplications.

Pultrusion manufacturing is a known process for producing high-strengthstructures. During pultrusion manufacturing, individual fiber strands,braids of strands, and/or woven fabrics are coated with or otherwiseimpregnated with a liquid matrix (e.g., a thermoset resin or a heatedthermoplastic) and pulled through a stationary die where the liquidmatrix cures and hardens into a final form. As with extrusionmanufacturing, UV light and/or ultrasonic vibrations are used in somepultrusion applications to speed the cure of the liquid matrix as itexits the die. The structures produced by the pultrusion manufacturingprocess have many of the same attributes of extruded structures, as wellas increased strength due to the integrated fibers. Although pultrusionmanufacturing can be used to continuously manufacture high-strengthstructures, the resulting structures may lack the form (shape, size,precision, and/or surface texture) required for some applications. Inaddition, conventional pultrusion manufacturing may lack precise controlover curing and the ability to dynamically change materials in thecomposite material during manufacture. Further, the variety of patternsand shapes integrated within the pultruded structures may be limited,thereby limiting available characteristics of the resulting structures.

Continuous fiber 3D printing (a.k.a., CF3D™) has recently been developedto address the shortcomings of extrusion and pultrusion manufacturing.CF3D involves the use of continuous fibers embedded within a matrixdischarging from a moveable print head. The matrix can be a traditionalthermoplastic, a powdered metal, a liquid resin (e.g., a UV curableand/or two-part resin), or a combination of any of these and other knownmatrixes. Upon exiting the print head, a head-mounted cure enhancer(e.g., a UV light, an ultrasonic emitter, a heat source, a catalystsupply, etc.) is activated to initiate and/or complete curing of thematrix. This curing occurs almost immediately, allowing for unsupportedstructures to be fabricated in free space. When fibers, particularlycontinuous fibers, are embedded within the structure, a strength of thestructure may be multiplied beyond the matrix-dependent strength. Anexample of this technology is disclosed in U.S. Pat. No. 9,511,543,which issued to TYLER on Dec. 6, 2016.

Although CF3D™ provides for increased strength, compared tomanufacturing processes that do not utilize continuous fiberreinforcement, improvements can be made to the structure and/oroperation of existing systems. The disclosed additive manufacturingsystem and print head are uniquely configured to provide theseimprovements and/or to address other issues of the prior art.

The disclosed system is directed to addressing one or more of theproblems set forth above and/or other problems of the prior art.

SUMMARY

In one aspect, the present disclosure is directed to a head for anadditive manufacturing system. This head may include a matrix reservoirconfigured to hold a supply of matrix, and a nozzle connected to an endof the matrix reservoir. The head may also include a plurality ofsensors located at an end of the matrix reservoir opposite the nozzle.

In another aspect, the present disclosure is directed to a head for anadditive manufacturing system. This head may include a matrix reservoirconfigured to hold a supply of matrix, and a nozzle connected to an endof the matrix reservoir. The head may also include a saturation sensorconfigured to generate a signal indicative of an amount of matrixsaturated within a reinforcement passing through the nozzle.

In yet another aspect, the present disclosure is directed to a head foran additive manufacturing system. This head may include a matrixreservoir configured to hold a supply of matrix, and a nozzle connectedto an end of the matrix reservoir. The head may also include a firstmatrix sensor located at the first end of the matrix reservoir, and asecond matrix sensor located at the second end of the matrix reservoir.

In still another aspect, the present disclosure is directed to a systemfor additively manufacturing a composite structure. The system mayinclude a head configured to discharge a continuous reinforcement atleast partially coated with a matrix. The head may have a matrixreservoir, and a nozzle connected to an end of the matrix reservoir. Thesystem may further include a support configured to move the head duringdischarging, and a supply of matrix. The system may also include atleast one sensor configured to generate a signal indicative of a matrixcharacteristic inside of the head, and a controller configured toselectively affect the supply of matrix based on the signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed additivemanufacturing system; and

FIGS. 2 and 3 are cross-sectional side- and end-view illustrations,respectively of an exemplary disclosed print head that may be used inconjunction with the system of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary system 10, which may be used tocontinuously manufacture composite structures 12 having any desiredcross-sectional shape (e.g., circular, rectangular, or polygonal).System 10 may include at least a support 14 and a head 16. Head 16 maybe coupled to and moved by support 14. In the disclosed embodiment ofFIG. 1, support 14 is a robotic arm capable of moving head 16 inmultiple directions during fabrication of structure 12, such that aresulting longitudinal axis (e.g., a trajectory) of structure 12 isthree-dimensional. Support 14 may alternatively embody an overheadgantry or a hybrid gantry/arm also capable of moving head 16 in multipledirections during fabrication of structure 12. Although support 14 isshown as being capable of 6-axis movements, it is contemplated that anyother type of support 14 capable of moving head 16 in the same or adifferent manner could also be utilized. In some embodiments, a drivemay mechanically couple head 16 to support 14, and include componentsthat cooperate to move portions of and/or supply power to head 16.

Head 16 may be configured to receive or otherwise contain a matrixmaterial. The matrix material may include any type of matrix material(e.g., a liquid resin, such as a zero-volatile organic compound resin, apowdered metal, etc.) that is curable. Exemplary resins includethermosets, single- or multi-part epoxy resins, polyester resins,cationic epoxies, acrylated epoxies, urethanes, esters, thermoplastics,photopolymers, polyepoxides, thiols, alkenes, thiol-enes, and more. Inone embodiment, the matrix material inside head 16 may be pressurized,for example by an external device (e.g., by an extruder or another typeof pump—not shown) that is fluidly connected to head 16 via acorresponding conduit (not shown). In another embodiment, however, thepressure may be generated completely inside of head 16 by a similar typeof device. In yet other embodiments, the matrix material may begravity-fed into and/or through head 16. For example, the matrixmaterial may be fed into head 16, and pushed or pulled out of head 16along with one or more continuous reinforcements. In some instances, thematrix material inside head 16 may need to be kept cool and/or dark inorder to inhibit premature curing or otherwise obtain a desired rate ofcuring after discharge. In other instances, the matrix material may needto be kept warm for similar reasons. In either situation, head 16 may bespecially configured (e.g., insulated, temperature-controlled, shielded,etc.) to provide for these needs.

The matrix material may be used to coat any number of continuousreinforcements (e.g., separate fibers, tows, rovings, socks, and/orsheets of continuous material) and, together with the reinforcements,make up a portion (e.g., a wall) of composite structure 12. Thereinforcements may be stored within (e.g., on one or more separateinternal spools—not shown) or otherwise passed through head 16 (e.g.,fed from one or more external spools 17—shown only in FIG. 2). Whenmultiple reinforcements are simultaneously used, the reinforcements maybe of the same material composition and have the same sizing andcross-sectional shape (e.g., circular, square, rectangular, etc.), or adifferent material composition with different sizing and/orcross-sectional shapes. The reinforcements may include, for example,carbon fibers, vegetable fibers, wood fibers, mineral fibers, glassfibers, metallic wires, optical tubes, etc. It should be noted that theterm “reinforcement” is meant to encompass both structural andnon-structural types of continuous materials that are at least partiallyencased in the matrix material discharging from head 16.

The reinforcements may be exposed to (e.g., at least partially coatedwith) the matrix material while the reinforcements are inside head 16,while the reinforcements are being passed to head 16, and/or while thereinforcements are discharging from head 16. The matrix material, dryreinforcements, and/or reinforcements that are already exposed to thematrix material may be transported into head 16 in any manner apparentto one skilled in the art. In some embodiments, a filler material (e.g.,chopped fibers) may be mixed with the matrix material before and/orafter the matrix material coats the continuous reinforcements.

One or more cure enhancers (e.g., a UV light, an ultrasonic emitter, alaser, a heater, a catalyst dispenser, etc.) 18 may be mounted proximate(e.g., within, on, or adjacent) head 16 and configured to enhance a curerate and/or quality of the matrix material as it is discharged from head16. Each cure enhancer 18 may be controlled to selectively exposeportions of structure 12 to energy (e.g., UV light, electromagneticradiation, vibrations, heat, a chemical catalyst, etc.) during theformation of structure 12. The energy may increase a rate of chemicalreaction occurring within the matrix material, sinter the material,harden the material, or otherwise cause the material to cure as itdischarges from head 16. In the depicted embodiments, cure enhancer 18includes multiple LEDs (e.g., 6 different LEDs) that are equallydistributed about a center axis of head 16. However, it is contemplatedthat any number of LEDs or other energy sources could alternatively beutilized for the disclosed purposes and/or arranged in another manner(e.g., unequally distributed). The amount of energy produced by cureenhancer 18 may be sufficient to cure the matrix material beforestructure 12 axially grows more than a predetermined length away fromhead 16. In one embodiment, structure 12 is completely cured before theaxial growth length becomes equal to an external diameter of thematrix-coated reinforcement. In another embodiment, only an outer shellof structure 12 is cured before the axial growth length becomes equal toan external diameter of the matrix-coated reinforcement.

The matrix material and/or reinforcement may be discharged from head 16via at least two different modes of operation. In a first mode ofoperation, the matrix material and/or reinforcement are extruded (e.g.,pushed under pressure and/or mechanical force) from head 16, as head 16is moved by support 14 to create the 3-dimensional trajectory within alongitudinal axis of structure 12. In a second mode of operation, atleast the reinforcement is pulled from head 16, such that a tensilestress is created in the reinforcement during discharge. In this mode ofoperation, the matrix material may cling to the reinforcement andthereby also be pulled from head 16 along with the reinforcement, and/orthe matrix material may be discharged from head 16 under pressure alongwith the pulled reinforcement. In the second mode of operation, wherethe matrix material is being pulled from head 16 with the reinforcement,the resulting tension in the reinforcement may increase a strength ofstructure 12 (e.g., by aligning the reinforcements, inhibiting buckling,compressing the matrix, ensuring distributed reinforcement loading,etc.), while also allowing for a greater length of unsupported structure12 to have a straighter trajectory. That is, the tension in thereinforcement remaining after curing of the matrix material may actagainst the force of gravity (e.g., directly and/or indirectly bycreating moments that oppose gravity) to provide support for structure12.

The reinforcement may be pulled from head 16, as a result of head 16moving away from an anchor point 20. In particular, at the start ofstructure formation, a length of matrix-impregnated reinforcement may bepulled and/or pushed from head 16, deposited onto anchor point 20, andcured such that the discharged material adheres (or is otherwisecoupled) to anchor point 20. Thereafter, head 16 may be moved away fromanchor point 20, and the relative movement may cause the reinforcementto be pulled from head 16. It should be noted that the movement ofreinforcement through head 16 could be assisted (e.g., via internal headmechanisms), if desired. However, the discharge rate of reinforcementfrom head 16 may primarily be the result of relative movement betweenhead 16 and anchor point 20, such that tension is created within thereinforcement. It is contemplated that anchor point 20 could be movedaway from head 16 instead of or in addition to head 16 being moved awayfrom anchor point 20.

A controller 22 may be provided and communicatively coupled with support14, head 16, and any number of cure enhancers 18. Each controller 22 mayembody a single processor or multiple processors that are configured tocontrol an operation of system 10. Controller 22 may include one or moregeneral or special purpose processors or microprocessors. Controller 22may further include or be associated with a memory for storing data suchas, for example, design limits, performance characteristics, operationalinstructions, tool paths, and corresponding parameters of each componentof system 10. Various other known circuits may be associated withcontroller 22, including power supply circuitry, signal-conditioningcircuitry, solenoid driver circuitry, communication circuitry, and otherappropriate circuitry. Moreover, controller 22 may be capable ofcommunicating with other components of system 10 via wired and/orwireless transmission.

One or more maps may be stored in the memory of controller 22 and usedduring fabrication of structure 12. Each of these maps may include acollection of data in the form of lookup tables, graphs, and/orequations. In the disclosed embodiment, the maps may be used bycontroller 22 to determine the movements of head 16 required to producethe desired size, shape, and/or contour of structure 12, and to regulateoperation of cure enhancers 18 (and/or other components of system 10) incoordination with the movements.

An exemplary head 16 is disclosed in detail in FIGS. 2 and 3. Head 16may include, among other things, a matrix reservoir 24 and a nozzle 26removably and fluidly connected to matrix reservoir 24. In this example,nozzle 26 is single path nozzle configured to discharge compositematerial having a generally circular cross-section. The configuration ofhead 16, however, may allow nozzle 26 to be swapped out for anothernozzle (not shown) that discharges composite material having a differentshape (e.g., a tubular cross-section, a linear cross-section, arectangular cross-section, a triangular cross-section, etc.).

An internal volume of matrix reservoir 24 may communicate with nozzle 26via a central opening 28. In the disclosed embodiment, matrix reservoir24 has a generally circular cross-section, and tapers radially inward tocentral opening 28. A size (e.g., diameter and/or height) of matrixreservoir 24 may be sufficient to hold a supply of matrix materialnecessary for wetting reinforcements passing through nozzle 26. Itshould be noted that matrix reservoir 24 could have a cross-sectionalshape other than circular, if desired.

During operation of head 16, care should be taken to ensure that anadequate amount of the matrix material (e.g., enough matrix material toensure a minimum saturation level of the associated reinforcements) isalways within matrix reservoir 24. In one embodiment, the matrixmaterial is supplied to matrix reservoir 24 by way of a supply 32 and/ora valve 34. Supply 32 may be, for example, a pump or an elevated tankthat generates a pressure (e.g., in response to a signal from controller22) sufficient to cause the matrix material to flow into matrixreservoir 24. Valve 34 may be fluidly disposed between supply 32 andmatrix reservoir 24, and moveable (e.g., in response to a signal fromcontroller 22) to any position between a flow-passing position and aflow-blocking position to selectively meter the matrix material intomatrix reservoir 24. It is contemplated that valve 34 may be omitted, insome applications, with operation of supply 32 alone being sufficient tometer the matrix material into matrix reservoir 24.

The matrix material may be selectively metered (e.g., via operation ofsupply 32 and/or valve 34) in response to an indication of an amount ofmatrix material inside of matrix reservoir 24 and/or a saturation levelof the reinforcements passing through matrix reservoir 24 or dischargingfrom nozzle 26. This indication may be provided, for example, via one ormore sensors 36. For example, when sensor(s) 36 indicate that the amountof matrix material in reservoir 24 and/or a saturation level of thereinforcements is below a threshold amount/level, controller 22 maygenerate a command directed to supply 32 and/or valve 34, causingadditional matrix material to be metered into matrix reservoir 24. Andwhen sensor(s) 36 indicate that the amount of matrix material inreservoir 24 and/or a saturation level of the reinforcements is at orabove the threshold amount/level, controller 22 may stop generating thecommand (or alternatively generate a stop command), causing metering ofthe matrix material to be halted.

In one embodiment, four different sensors 36A are utilized and placedaround an upper end of matrix reservoir 24 (e.g., at an end oppositenozzle 26). These sensors 36A may be annularly distributed about aperiphery of matrix reservoir 24, and separated by an angle α (shown inFIG. 3). In one example, angle α is about (e.g., within engineeringtolerances of) 90°. In this embodiment, sensors 36A may be, for example,LIDAR, RADAR, ultrasonic, infrared, and/or stereoscopic optical-typesensors that are configured to detect a surface level location of thematrix material. The annular distribution of sensors 36A may allow fordetection of the surface level location regardless of tilting of head16. In this arrangement, signals from one or more of sensors 36A may beused together to determine the amount of matrix material inside ofmatrix reservoir 24 at any given time. It is contemplated that a greateror lesser number (e.g., three) sensors could be utilized, as desired.

In another embodiment, a single sensor 36B is located at a lower end ofmatrix reservoir 24 (e.g., adjacent an inlet to nozzle 26) and used inconjunction with one or more sensors 36A located at an upper end ofmatrix reservoir 24. In this embodiment, sensors 36 may be, for example,contact sensors, which are configured to generate signals indicative ofdirect contact being made with the matrix material.

In yet another embodiment, one or more acoustic-type sensors 36C couldbe located anywhere within head 16. In this embodiment, sensor(s) 36 maygenerate a vibration within head 16, and receive back a correspondingreflection of this vibration (e.g., a standing wave). A comparison ofthe generated vibration and the received reflection may provide anindication of an amount of the matrix material inside of matrixreservoir 24.

In a final embodiment, one or more saturation sensors 36D may be locatedat the lower end of matrix reservoir 24 (e.g., inside of opening 28and/or nozzle 26) and configured to determine an amount of the matrixmaterial saturated into the reinforcement just prior to thereinforcement entering nozzle 26. Saturation sensor(s) 36 may be, forexample, light sensors, current sensors, cameras, vibration sensors,and/or another type of sensor that directly measures a characteristic ofthe matrix-coated reinforcement (e.g., an opacity, a resistance, anappearance, a resonance, etc.) for comparison with expectedcharacteristics of a sufficiently saturated reinforcement. Controller 22may thereafter generate the commands discussed above based on thecomparison.

INDUSTRIAL APPLICABILITY

The disclosed system and print head may be used to continuouslymanufacture composite structures having any desired cross-sectionalshape and length. The composite structures may include any number ofdifferent fibers of the same or different types and of the same ordifferent diameters, and any number of different matrixes of the same ordifferent makeup. Operation of system 10 will now be described indetail.

At a start of a manufacturing event, information regarding a desiredstructure 12 may be loaded into system 10 (e.g., into controller 22 thatis responsible for regulating operations of support 14 and/or head 16).This information may include, among other things, a size (e.g.,diameter, wall thickness, length, etc.), a contour (e.g., a trajectory),surface features (e.g., ridge size, location, thickness, length; flangesize, location, thickness, length; etc.), connection geometry (e.g.,locations and sizes of couplings, tees, splices, etc.), desired weavepatterns, weave transition locations, etc. It should be noted that thisinformation may alternatively or additionally be loaded into system 10at different times and/or continuously during the manufacturing event,if desired. Based on the component information, one or more differentreinforcements and/or matrix materials may be selectively installedand/or continuously supplied into system 10.

To install the reinforcements, individual fibers, tows, and/or ribbonsmay be passed through matrix reservoir 24 and nozzle 26. In someembodiments, the reinforcements may also need to be connected to apulling machine (not shown) and/or to a mounting fixture (e.g., toanchor point 20). Installation of the matrix material may includefilling head 16 (e.g., reservoir 24) and/or coupling of an extruder (notshown) to head 16.

The component information may then be used to control operation ofsystem 10. For example, the reinforcements may be pulled and/or pushedalong with the matrix material from head 16. Support 14 may alsoselectively move head 16 and/or anchor point 20 in a desired manner,such that an axis of the resulting structure 12 follows a desiredthree-dimensional trajectory. Cure enhancers 18 may be adjusted duringoperation to provide for desired curing conditions. Once structure 12has grown to a desired length, structure 12 may be severed from system10.

During operation, as matrix-wetted reinforcements are being discharged(e.g., pushed and/or pulled) from nozzle 26, a level of the matrixwithin reservoir 24 may lower. If unaccounted for, the reinforcementsmay eventually be discharged with less matrix than desired. Accordingly,as the level of matrix lowers and/or as the matrix-to-fiber ratio ofdischarging material is reduced, sensor(s) 36 may generate signalsindicative of the amount of matrix within reservoir 24 and/or the ratio.These signals may be received and interpreted by controller 22, andcontroller 22 may selectively respond to the signals. For example,controller 22 may compare the actual level of matrix within reservoir 24and/or the actual matrix-to-fiber ratio to one or more threshold (e.g.,minimum and/or maximum) levels. As the actual level and/or ratioapproaches and/or passes through the threshold level(s), controller 22may selectively adjust operation of supply 32 and/or valve 34 to eitherinitiate or increase matrix flow into reservoir 24 or to slow or stopthe matrix flow.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed system andprint head. Other embodiments will be apparent to those skilled in theart from consideration of the specification and practice of thedisclosed system and print head. It is intended that the specificationand examples be considered as exemplary only, with a true scope beingindicated by the following claims and their equivalents.

What is claimed is:
 1. A print head for an additive manufacturingsystem, comprising: a matrix reservoir configured to hold a supply ofmatrix; a nozzle connected to an end of the matrix reservoir; and aplurality of sensors located at an end of the matrix reservoir oppositethe nozzle.
 2. The print head of claim 1, wherein the plurality ofsensors are distributed around a perimeter of the matrix reservoir. 3.The print head of claim 2, wherein an angle between adjacent sensors ofthe plurality of sensors is about 90°.
 4. The print head of claim 2,wherein the plurality of sensors are fluid level sensors.
 5. The printhead of claim 4, wherein the plurality of sensors are acoustic sensors.6. A print head for an additive manufacturing system, comprising: amatrix reservoir; a nozzle connected to an end of the matrix reservoir;and a saturation sensor configured to generate a signal indicative of anamount of matrix saturated within a reinforcement passing through thenozzle.
 7. The print head of claim 6, wherein the saturation sensor islocated between the end of the matrix reservoir and the nozzle.
 8. Aprint head for an additive manufacturing system, comprising: a matrixreservoir having a first end and a second end; a nozzle connected to thefirst end of the matrix reservoir; a first matrix sensor located at thefirst end of the matrix reservoir; and a second matrix sensor located atthe second end of the matrix reservoir.
 9. The print head of claim 8,wherein the first and second matrix sensors are contact sensors.
 10. Anadditive manufacturing system, comprising: a head configured todischarge a continuous reinforcement at least partially coated with amatrix, the head including: a matrix reservoir; and a nozzle connectedto an end of the matrix reservoir; a support configured to move the headduring discharging; a supply of matrix; at least one sensor configuredto generate a signal indicative of a matrix characteristic inside of thehead; and a controller configured to selectively affect the supply ofmatrix based on the signal.
 11. The additive manufacturing system ofclaim 10, wherein: the supply of matrix includes a pump; and thecontroller is configured to selectively activate the pump based on thesignal.
 12. The additive manufacturing system of claim 10, wherein: thesupply of matrix includes a pump and a valve disposed between the pumpand the matrix reservoir; and the controller is configured toselectively activate the valve based on the signal.
 13. The additivemanufacturing system of claim 10, wherein the matrix characteristic is alevel of matrix.
 14. The additive manufacturing system of claim 10,wherein the matrix characteristic is a saturation amount of areinforcement passing through the head.
 15. The additive manufacturingsystem of claim 10, wherein: the at least one sensor includes pluralityof sensors distributed around a perimeter of the matrix reservoir; andthe controller is configured to determine a level of matrix inside ofthe matrix reservoir based on signals generated by all of the pluralityof sensors.
 16. The additive manufacturing system of claim 15, whereinthe plurality of sensors are acoustic sensors.
 17. The additivemanufacturing system of claim 16, wherein an angle between adjacentsensors of the plurality of sensors is about 90°.
 18. The additivemanufacturing system of claim 10, wherein: the at least one sensorincludes a first sensor located at a first end of the matrix reservoiraway from the nozzle, and a second sensor located at a second end of thematrix reservoir adjacent the nozzle; and the controller is configuredto determine a level of matrix inside of the matrix reservoir based onsignals generated by both of the first and second sensors.
 19. Theadditive manufacturing system of claim 18, wherein the first and secondsensors are contact sensors.
 20. The additive manufacturing system ofclaim 10, wherein the at least one sensor includes a saturation sensorlocated between the matrix reservoir and the nozzle.